MEE 352 – Analysis and Design of Composite Structures
Mark Jacobs, and Stephen Knittweis
What follows will be an informal recounting, rather than a formal report, of the engineering challenges, the design decisions to overcome them and the techniques learned as a result during the course of my project to build a fiberglass balloon animal.
When the project was first assigned I had no ideas for what to make. I knew I didn’t want to make something so simple that it would essentially be a cop out and hardly give me any experience with the materials. Since the requirements were open-ended, I wanted to make something light-hearted and fun as well, to mix the comical with serious design.When Steve suggested we make a pressure vessel by wrapping fiberglass around a balloon, inspiration struck in the form of a pressure vessel in the shape of a balloon animal. A little more discussion later we decided to shift from a seriously functional pressure vessel to a piece blending engineering design with art. We would keep the pressure fitting in the piece but it would be more for appearances than actual use. And then I spoke one of the most dangerous sentences in the history of civilization; “After all, how hard can it be?”
After my initial exuberance wore off, I realized that I didn’t know how to make balloon animals.
Challenge – The first and simplest challenge was to learn how to make a balloon animal. Solution – A quick internet search and a visit to the party store gave me a “balloon animal starter kit” and I set in practicing making the star of the project.Challenge – How would the mold be designed to get a full coverage mold that would yield a product that retained the characteristic look.
Solution – A two-part mold was made using the cardboard from soda containers, old sales flyers and water thinned white glue. This created a permanent mold not prone to the shrinkage of the balloon animal from which it was made.
Challenge – Now we have a fragile, but stable and detailed negative mold to work with, the question is how can the mold be reinforced so that it may be worked with freely without fear of damaging it. The danger was that the mold was fragile enough that there was a real chance that the reinforcement effort could crush or damage it during the effort.Solution – Inspiration was found in the form of the paper in my household shredder, by combining the shreds with thinned white glue we could make a loose fiber reinforcement structure, contained by cardboard walls, that would be strong enough to protect the mold but not so heavy that it would collapse it while drying.
Challenge – The molds were detailed enough that the walls had been built around the ears and legs tightly enough to create reverse angles. It became evident that it would be extremely difficult to apply fiberglass to these tight spaces. Even if we were able to apply the glass successfully, it was likely the mold would need to be destroyed to remove it.
Solution – After applying carnauba wax as a mold release agent, a silicone mold making material was painted into the mold. The goal was to create a flexible positive mold that could be removed from the negative mold without destroying either.
Unfortunately, the silicone ran and pooled in the bottom of the mold, leaving the sides tissue thin and unable to withstand being peeled out of the mold. Applying multiple layers to build up thickness was impractical due to time constraints, and the mold material was too expensive to use to completely fill the mold for a solid casting.
Challenge – At this point we were offered access to a reusable mold material called ComposiMold made by a local company collaborating with Campus Ventures. The ComposiMold was perfect for what we needed to do, but we still had the challenge of the reverse angles and how to remove the positive mold after it cured.
Solution – I used my Dremel to remove the overhangs of the reverse angles, and then filled the holes caused by that removal with spackle. A thinned coat of white glue was applied to re-seal the porous surfaces of the patched spots.Now that the mold had straight sides the next challenge was to devise a way to help facilitate pulling the ComposiMold out after it cured. Nail heads suspended from small pieces of wood were placed at regular intervals to act as anchor-points providing additional leverage during de-molding.
Now with the ComposiMold in hand, the project starts to pick up speed. It melts readily in the microwave when heated in 30 second bursts and stirred in between heatings. It poured smoothly and spread evenly throughout the mold.Twenty-four hours later it pulled easily from the mold with minimal effort. The pieces of wood were carefully removed and the nails nipped off at the surface.
Now with a positive mold laying the fiberglass became dramatically easier. The random fiber direction “cat fur” fiberglass was used for its ease of pressing it into tight corners and getting good coverage of complex curves. The loose cross-weave fabric was much more convenient to use and held its shape better than the cat fur and was used to cover holes and wrap around the figure to seal the two halves together.
The ComposiMold pulled out of the fiberglass shell just as easily as the papier mache mold. The ComposiMold was then re-melted, after the nail heads were removed, and re-used for the second half of the balloon animal.
Each half was trimmed, sanded and dressed in preparation for being mated together. The next step was to build up a base around each leg to allow it to be free-standing. I rolled strips of fabric into “logs” before saturating them in the matrix and wrapping it around the feet to build up bases. Two blocks were wrapped in the white silicone-coated, non-stick paper to support the model while the bases dried.
Strips of loose cross-weave fiberglass were wrapped around the piece joining the two halves together and sealing any holes that remained in the structure of the piece.
I am pleased with the final result and think that we were successful but even more than that I’m pleased with the journey of challenge and discovery that we worked our way through. I learned how to make a balloon animal, how to work with papier mache, the utility of ComposiMold and different characteristics of fiberglass weaves and some do’s and don’t’s of mixing up resin.
My secondary goal was also successful. The balloon project got people talking and got people interested. At one point in the shop, someone who I had never met came in and said “Is that the balloon animal? I heard about that.” That kind of buzz can only be good for morale and can help to keep people engaged.
www.ComposiTherm.com to order